Process for the recovery of benzene



J1me 1957 A. SCHMALENBACH EI'AL 2,794,515

PROCESS FOR THE RECOVERY 0F BENZENE Filed Jan. 12, 1954 JNVEMURS. 40a; F Joya-946M940! He:

57 196M5 5; aka/V0 55.

off/ 4M rron/wax United States Patent PROCESS FOR THE RECOVERY OF BENZENE Adolf Schmalenbach, Herbert Pummer and Herbert Brandes, Essen, Germany Application January 12, 1954, Serial No. 403,534

Claims. 01. 183-115) Numerous proposals have been made for the removal 1 of benzene hydrocarbons and naphthalene, for example from coke oven gas, and some of these have been of considerable technical importance. Thus a process which is widely carried out in modern coking industry, I.

and involves the production of by-products, resides in bringing the coke oven gas at normal or even elevated pressure into contact with benzene wash oil, that is to say oil of a specific tar fraction, the benzene hydrocarbons Which dissolve in the wash oil being subsequently recovered from the latter by heating. This method prov-ides for a very extensive reduction in the benzene content of the gas, but the cost of a plant for this purpose is comparatively high and it is desirable that this cost be reduced.

It has also been proposed to apply very extreme cooling to the benzene-containing gas, for example to temperatures of the order of 40 C., and to freeze out benzene hydrocarbons, in this way. Quite apart from hydrocarbon content of the gas and the degree to which the removal of benzene is to be carried. The process can be carried out at more or less normal pressure, but it is also possible to use high or very high pressures, according to whether the purified coke oven gas is, for ex ample, to be used as local town gas or for long-distance distribution.

A preferred form of the method according to the invention resides in cooling the gas, after prior cooling below normal temperature, with a cooled concentrated calcium chloride solution, brought into the cycle, to temperatures below 0 C. and concomitantly introducing into this cooling stage the wash solution containing benzene hydrocarbons which is obtained from the Washing of the gas with the solvent for the benzene.

' One form of plant for carrying out the method of this invention is diagrammatically illustrated in the drawing. This plant serves to remove benzene from a gas having a benzene hydrocarbon content of about 30 grams per normal cubic meter and a naphthalene content of about 0.8 gram per normal cubic meter to a residual benzene content of 0.1 gram per normal .cubic meter, and at the same time to render it practically completely free of naphthalene.

The gas to be freed from benzene passes through a pipe 1 into a compressor 2 in which it is compressed to 120 pounds per square inch. The gas,rheated as a result of this compression, passes through'a'pipe 3 into the-cooler stage I, represented by anlindirect water cooler 4. A condensate at a temperature of about 25 C.

is is formed in the cooler 4, about two-thirds of this being the fact that the production of such low temperatures 1 peratures, so that after a specific period the plant must be cleared out, additional energy being required for this purpose.

It is an object of the present invention to improve, and in particular to reduce the cost of, the existing methods and plants for the separation of benzene hydrocarbons frorn gases. This aim is basically attained by the present invention in that the gas containing the benzene hydrocarbons, for example coke oven gas, after leaving a final cooler is first subjected to temperature and pressure conditions such that the higher boiling benzene hydrocarbons, and naphthalene, are practically completely condensed, while, on the other hand, the lower boiling benzene hydrocarbons are at least partially condensed. The gas, which now substantially only contains low boiling benzene hydrocarbons, is then washed in counter-flow with a solvent for benzene hydrocarbons, consisting of the higher boiling but naphthalene-free fraction of the condensate produced in the process itself, which is introduced into the circuit.

It is possible, using the method according to this invention to remove benzene continuously from the gas without cooling the benzene to thedegree required in the modern technique of by-products recovery.

The pressure and temperature conditions which are used for the purpose indicated depend on the benzene benzene hydrocarbons. The remainder comprises solvent naphtha and the major part of the naphthalene con tained in the gas. Y

This condensate flows through a pipe 5 into a condensate container 6. The gas leaves the cooler 4 through a pipe 7 and passes to the cooling stage II, consisting generally of an indirect cooler 8, in which the gas is brought into indirect heat exchange relationship with cold gas from which benzene has been removed and derived from a later stage in the operation. In this cooling stage the temperature of the gas is lowered to about 3 C. and it then passes through a pipe 9 into a combined Washing and cooling column 10. The condensate formed in the cooler 8 consists of more than percent benzene hydrocarbons together with some solvent naphtha, and the remainder of the naphthalene. 5 The condensate flows through a pipe 8a into a condensate container 11 and can be withdrawn from this through a pipe 12 as crude benzene for use for any suitable purpose.

The gas cooled to a temperature of about 3 C. passes into the bottom of the column 10 and is there treated simultaneously with a cold concentrated calcium chloride solution, which passes through a pipe 13 located approximately at the intermediate portion of the column, and cold solvent naphtha which is passed through a pipe 14 into the head of the column. The temperature of the calcium chloride solution and of the solvent naphtha is about l6 C. The gas, practically completely freed of benzene, leaves the washer through a pipe 15 and may be withdrawn through apipe 16 after flowing through the cooler 8. The discharge from the column 10 passes through a pipe 17 into a Wash condensate container 18, Where separation of the liquid mixture takes place such that the concentrated chloride solution separates out at the bottom of the container at a temperature of 8.5 C., while the benzene-containing solvent naphtha forms a layer on the calcium chloride solution, this layer then passing over a weir 19 into a drain pipe 20. The calcium chloride solution is withdrawn from the condensate container 18 through a pipe 21 by means of a pump 22 and is forwarded through a pipe 23 to a cooler plant 24, from where it passes at a temperature of about l6 C. through pipe 13 into the column 10. The cooler plant operates on the known ammonia-compression principle. An ammonia compressor 25 is utilized for this purpose, the ammonia which expands in the cooler plant being recompressed in this compressor after passing through the heat exchanger 26. When the feed gases contain water vapor, such as does coke oven gas, and an aqueous condensate is formed in the column, apart of the calcium chloride solution with- ,drawn from the condensate container 18 is supplied through a valve 27 and a pipe '23 to a heat exchanger 29 and from there through a pipe 30 to avaporizing device 31 which is operated with indirect steam and in which steam is evaporated from the calcium chloridesolution flowing therethrough to such an extent that the concentration of the calcium chloride solution, which was lowered by the absorption of water in the washing column 10, isrestored to its original concentration. After this concentration has been increased, the calciumchloride solution passes through a pipe 32 through the heat exchanger 29 and a pipe 33 back to the condensate container 18.

While the condensate is being removed from the container 11 through pipev 12 directly for use as crude benzene, the condensate is supplied from the container 6, through a pipe 34, a pump 35, a pipe 36, and the discharge from the washing condensate container 18.through the pipe 20, a pump 37, and a pipe 38, to a distillation column 39. a

The condensate from the condensate container 6 flows from pipe 36 first to a heat exchanger 40 in which its temperature is raised from about 25 C. to about 138 C., and from there it is passed through a pipe 41 into the middle part of the distillation column 39. The condensate from the wash condensate container 18 passes from the pipe 38 first into a heat exchanger 42 and then through a pip'efl43 to the head of column 39 and there i V servesrso to-speak,'as a reflux component. During this passage through the heat exchanger 42, the temperature of the washing condensate is increased from about 8.4 C. to 73 C. r

The distillation column 39, which is heated by a circulatory heater 44, produces at its overhead a fraction consisting primarily, of benzene hydrocarbons with a small proportion of solvent naphtha. This fraction is condensed in the condensers 45 and may then, if desired, be combined with the condensate from the condensate container 11 for use as a crude benzene. A solvent naphtha fraction is withdrawn from a central lower part a of the distillation column 39 and is passed through a pipe 46 to the upper tray of a still 47 heated by a circulatory heater 48. The vapours from the still are returned through a'pipe 49 to column 39, while the heavier'prodnot, which comprises more than 95 percent solvent naphtha, is withdrawn through a pipe 50 and passed by a.

solvent naphtha can be withdrawn through a pipe 56 and a pipe 57 either into the condensate container 6 or by a pipe 58 to a collector for crude benzene.

Example 1 7 'It is' desired to remove benzene hydrocarbons from a coke oven gas produced at a rate of 1,000,000 Nm. (normal cubic meters).per day. The ratio of the benzene 4 hydrocarbons to the total quantity of benzene hydrocarbons (30 g./Nm. consists of:

% benzene 16% toluol 7% xylol 7% solvent naphtha In addition 73.3 kg. solvent naphtha and 31.6 kg. naphthalene are separated ofi? per hour in this cooler.

In the cooler stage H, in which the gas is cooled down from 25 C. to 3 C. there is produced, hourly, a condensate containing approximately: s

300 kg. benzene 102 kg. toluol 30.8 kg. xylol 13.9 kg. solvent naphtha and also 0.24 kg. naphthalene After leaving the column 10, which is supplied with 250 kg. solvent naphtha and 57 m. calcium chloride solution per hour the gas still has a residual benzene content of 0.2 g./Nm. The runoif from the column 10 comprises, in addition to calcium chloride solution,

479 kg. benzene 43 kg. toluol 16 kg. xylol and 233 kg. solvent naphtha It will be observed from these figures that during the washing procedure some solvent naphtha is given up to the gas, andfor this reason, as will already be apparent from the specific description above, it will not be sufiicient to treat solely the discharge from the column 10 to enable a constant quantity of solvent naphthato be supplied to this column 10. In fact, for this purpose the condensate must be taken from the first cooling stage in order to compensate for the loss to the gas of solvent naphtha.

The product obtained as overhead from the'distillation column 39 is a benzene fraction consisting of:

571 kg. benzene 98 kg. toluol 59 kg. xylol 35 kg. solvent naphtha per hour The discharge from the distillation column consists of 32 kg. solvent naphtha and 32 kg. naphthalene per hour. 11 kg. xylol and 239 kg. solvent naphtha at a temperature of C. can be withdrawn per hour from the still 47.

The main expenditure of energyin carrying out the method of this invention is accounted for by the .compression which takes place in the ammonia compressor 25. Further energy in the form of indirect steam is expended in heating the distillation column 39 and the still 47, and also the vaporizer '31.

We claim:

1. A process for separating benzene hydrocarbons and naphthalene from gases containing these materials and also containing water vapor, which comprises cooling said gases in a first stage to obtain a condensate containing substantially all of the higher boiling benzene hydrocarbons and the naphthalene'and a portion of the lower boiling benzene hydrocarbons originally present in said gases, separating the gases from the condensate, further cooling the gases separate from the condensate from the first stage in a second stage to a degree such that a condensate constituted primarily of the lower boiling benzene hydrocarbons and substantially free of naphthalene is produced, separating the further cooled gases from the condensate, contacting the gases separate from the condensate from the second stage in a cooling and washing zone with a cold, concentrated calcium chloride solution while concomitantly washing said gases in counterflow in said zone with a cold solvent for benzene hydrocarbons, the temperature of said calcium chloride solution and said solvent for benzene hydrocarbons being substantially below (3., whereby the temperature of the gases is lowered to substantially below 0 0., lower boiling benzene hydrocarbons are dissolved in said solvent and water combines with said calcium chloride solution, and thereafter separating said solvent having the lower boiling benzene hydrocarbons dissolved therein from said calcium chloride solution having water combined therewith.

2. A continuous process for separating benzene hydrocarbons and naphthalene from gases containing these materials and also containing water vapor, which comprises cooling said gases in a first stage to obtain a condensate containing substantially all of the higher boiling hydrocarbons and a portion of the lower boiling benzene hydrocarbons originally present in said gases, separating the gases from the condensate, further cooling the gases separate from the condensate from the first stage in a second stage to a degree such that a condensate constituted primarily of the lower boiling benzene hydrocarbons and substantially free of naphthalene is produced, separating the further cooled gases from the condensate, contacting the gases separate from the condensate from the second stage in a cooling and washing zone in counterflow therein with a cold, concentrated calcium chloride solution while concomitantlywashing said gases also in counterflow therein with a cold solvent for benzene hydrocarbons, the temperature of said calcium chloride solution and said solvent being substantially below 0 C., whereby the temperature of the gases is lowered to substantially below 0 C., lower boiling benzene hydrocarbons are dissolved in said solvent, and water combines with said calcium chloride solution, withdrawing cooled gases substantially free of benzene hydrocarbons from said cooling and washing zone, withdrawing a liquid mixture of solvent having the lower boiling benzene hydrocarbons dissolved therein and calcium chloride solution having water combined therewith from the last-mentioned zone and passing this liquid mixture to a separating zone, stratifying said liquid mixture in said separating zone to obtain a bottom layer of calcium chloride solution having water combined therewith and an upper layer of solvent having benzene hydrocarbons dissolved therein on said bottom layer, separating said upper layer of solvent having benzene hydrocarbons dissolved therein from said bottom layer, and thereafter recovering benzene hydrocarbons from the solvent having benzene hydrocarbons dissolved therein by distillation.

3. A continuous process in accordance with claim 2 wherein the gases containing the benzene hydrocarbons, naphthalene and water vapor are coke oven gases from a final cooler for coke oven gases.

4. A continuous process for separating benzene hydro carbons and naphthalene from gases containing these materials and also containing water vapor, which comprises cooling said gases in a first stage to obtain a condensate containing substantially all of the higher boiling benzene hydrocarbons and the naphthalene, and a portion of the lower boiling benzene hydrocarbons and solvent naphtha originally present in said gases, separating the gases from the condensate, further cooling the gases separate from the condensate from the first stage in a second stage to a degree such that a condensate constituted primarily of the lower boiling benzene hydrocarbons and substantially free of naphthalene is produced, separating the further cooled gases from the condensate, introducing the gases separate from the condensate from the second stage to a washing column at the lower portion'thereof, introducing a cold solvent for benzene hy+ drocarbons to said washing column at the upper portion thereof, introducing a cold, concentrated calcium chloride solution to said washing column at an intermediate portion'thereof; to effect a direct contacting of the gases at first simultaneously in the lower portion of the wash ing column with the calcium chloride solution and the solvent for benzene hydrocarbons and subsequently in the upperportion of the washing column with the solvent for benzene hydrocarbons, the temperature of said calcium chloride solution and said solvent for benzene hydrocarbons being substantially below 0 0., whereby the temperature of the gases is lowered to substantially be low 0 C., lower boiling benzene hydrocarbons are dissolved in said solvent, and water combines with said calcium chloride solution, withdrawing cooled gases substantially free of benzene hydrocarbons from an upper portion of said column, withdrawing a liquid mixture of solvent having the lower boiling benzene hydrocarbons dissolved therein and calcium chloride having water combined therewith from the lower portion of said column and passing this liquid mixture to a separating zone, separating said liquid mixture in said separating zone into a bottom layer of calcium chloride solution and an upper layer of the solvent having the benzene hydrocarbons dissolved therein on said bottom layer, separating said upper layer from said bottom layer, passing said condensate separate from the gases from said first stage to an intermediate portion of a distillation column, distilling said condensate in said distillation column to obtain an overhead fraction constituted primarily of benzene hydrocarbons and a bottoms fraction comprising naphthalene and solvent naphtha, and passing solvent having benzene hydrocarbons dissolved therein from said upper layer in said separating zone to the upper portion of said distillation column to provide reflux for the condensate vapors distilling therein.

5. A continuous process for separating benzene hydrocarbons and naphthalene from coke oven gases from a final cooler for coke :oven gases and containing these materials and also containing water vapor, which comprises cooling said coke oven gases in a first stage to obtain a condensate containing substantially all of the higher boiling benzene hydrocarbons and the naphthalene, and a portion of the lower boiling benzene hydrocarbons and solvent naphtha originally present in said gases, separating the gases from the condensate, further cooling the gases separated from the condensate from the first stage in a second stage to a degree such that a condensate constituted primarily of the lower boiling benzene hydrocarbons and substantially free of naphthalene is produced, separating the further cooled gases from the condensate, introducing the gases separate from the condensate from the second stage to a washing column at the lower portion thereof, introducing cold solvent naphtha to said washing column at the upper portion thereof, introducing cold concentrated calcium chloride solution to said washing column at the intermediate portion thereof to effect a direct contacting of the gases at first simultaneously in the lower portion of the washing column with the calcium chloride solution and the solvent naphtha and subsequently in the upper portion of the washing column with the solvent naphtha, the temperature of said calcium chloride solution and said solvent naphtha being substantially below 0 C., whereby the temperature of the gases is lowered to substantially below 0 C., lower boiling benzene hydrocarbons are dissolved in said solvent naphtha, and water combines with said calcium chloride solution, withdrawing cooled gases substantially free of benzene hydnocarbons from the upper portion of said washing column, withdrawing a liquid mixture of solvent having the lower boiling benzene hydrocarbons dissolved therein and calcium chloride having water combined therewith from the lower portion of said washing column and passing the liquid mixture to a separating zone,

separating said liquid mixture in'said separatingrzone into a bottom layer of calcium chloride solution and an upper layer of solvent nephtha'having'thebenzene hydro carbons dissolved therein on said bottom layenpassing said'oondensate separate from the gases from the first stage to an intermediate portion of a distillation column, distilling said condensate in said distilling columnto obtain an overhead fraction consisting prirnarily of benzene hydrocarbons and a small amount of solvent naphtha and a bottoms fra etion comprising naphthalene and spl- Yent naphtha, separating the naphthalene from the solvent naphtha in said bottoms-fraction, and passing said solvent naphtha having benzene hydrocarbons dissolved therein froni said upper layer in said separating zone to the ppper portion of said distillation oolumn to provide reflux for the condensate vapors distilling therein. 

1. A PROCESS FOR SEPARATING BENZENE HYDROCARBONS AND NAPHTHALENE FROM GASES CONTAINING THESE MATERIAL AND ALSO CONTAINING WATER VAPOR, WHICH COMPRISES COOLING SAID GASES IN A FIRST STAGE TO OBTAIN A CONDENSSTE CONTAINING SUBSTANTIALLY ALL OF THE HIGHER BOILING BENZENE HYDROCAARBONS AND THE NAPHTHALENE AND A PORTION OF THE LOWER BOILING BENZENE HYDROCARBONS ORIGINALLY PRESENT IN SAID GASES, SEPARATING THE GASES FROM THE CONDENSATE, FURTHER COOLING THE GASES FROM THE CONDENSATE FROM THE FIRST STAGE IN A SECOND STAGE TO A DEGREE SUCH THAT A CONDENSATE CONSTITUTED PRIMARILY OF THE LOWER BOILING BENZENE HYDROCARBONS AND SUBSTANTIALLY FREE OF NAPHTHALENE ID PRODUCED, SEPARATING THE FURTHER COOLED GASES FROM THE CONDESATE, CONTACTING THE GASES SEPARATE FROM THE CONDENSATE FROM THE SECOND STAGE IN A COOLING AND WASHING ZONE WITH A COLD, CONCENTRATED CALCIUM CHLORIDE SOLUTION WHILE CONCOMITANTLY WASHING SAID GASES IN COUNTERFLOW IN SAID ZONE WITH A COLD SOLVENT FOR BENZENE HYDROCARBONS, THE TEMPERATURRE OF SAID CALCIUM CHLORIDE SOLUTION AND SAID SOLVENT FOR BENZENE HYDROCARBONS BEING SUBSTANTIALLY BELOW 0*C., WHEREBY THE TEMPERATURRE OF THE GASES IS LOWERED TO SUBSTANTIALLY BELOW 0*C,. LOWER BOILING BENZENE HYDROCARBONS ARE DISSOLVED IN SAID SOLVENT AND WATER COMBINES WITH SAID CALCIUM CHLORIDE SOLUTION, ANDF THEREAFTER SEPARATING SAID SOLVENT HAVING THE LOWER BOILING BENZENE HYDROCARBONS DISSOLVED THEREIN FROM SAID CALCIUM CHLORIDE SOLUTION HAVING WATER COMBINED THEREWITH. 